Гляньте логи почтового сервера.
Обычно gmail отправляет ошибочку:
Nov 22 12:52:51 <изменненый_IP> postfix/smtp[276168]: B49121086B7E: to=<почта>, relay=gmail-smtp-in.l.google.com[173.194.222.27]:25, delay=0.76, delays=0.02/0/0.18/0.56, dsn=5.7.25, status=bounced (host gmail-smtp-in.l.google.com[173.194.222.27] said: 550-5.7.25 [<изменненый_IP>] The IP address sending this message does not have a 550-5.7.25 PTR record setup, or the corresponding forward DNS entry does not 550-5.7.25 point to the sending IP. As a policy, Gmail does not accept messages 550-5.7.25 from IPs with missing PTR records. Please visit 550-5.7.25 https://support.google.com/mail/answer/81126#ip-practices for more 550 5.7.25 information. gsmtp (in reply to end of DATA command))